1. Field of the Invention
The present invention relates to a carrier for use in electrostatic image development performed in electrophotography, electrostatic recording, and electrostatic printing, etc., and further relates to a developer containing the carrier, an image forming apparatus and a process cartridge using the carrier or the developer.
2. Discussion of the Background
Image forming methods for use in electrophotography typically include the following processes:    (1) Forming a latent electrostatic image on an image bearing member formed of, for example, a photoconductive material;    (2) Attaching charged toner particles to the latent electrostatic image to form a visualized toner image;    (3) Transferring the visualized toner image to a recording material such as paper; and    (4) Fixing the visualized toner image on the recording material before discharging the recording material outside;
Recently, the technology for use in photocopiers and printers using electrophotography has been rapidly extending from monochrome photocopying or printing to color photocopying or printing. Therefore the full color photocopier and printer market is expanding.
In color image forming based on full color electrophotography, all colors are typically reproduced by overlapping the layers of the three primary color toners, i.e., yellow, magenta and cyan toner, or of four color toners including the three primary color toners and black toner. To obtain a vivid and clear full color image having a good reproducibility, it is necessary to reduce light scattering by smoothing the surface of a fixed toner image to some degree. Because of this, typical full color photocopiers, etc. produce images having a gloss in the medium to high range, i.e., 10 to 50% in most cases.
As a method of fixing a dry toner image on a recording medium, a contact heating fixing method in which a roller or a belt having a smooth surface is heated to fix toner upon application of heat and pressure is normally adopted in many cases. This method is thermally efficient and fixes toner at a high speed, thereby providing gloss and transparency to color toner. To contrary to this advantage, offset phenomenon, in which part of a toner image attaches to the surface of a fixing roller and transfers to another image, occurs because the surface of the heated fixing roller is press-contacted with melted toner before detachment.
To prevent this offset phenomenon, a countermeasure has been adopted in which the surface of a fixing roller is formed of a material having a good releasability such as silicone rubber and fluorine containing resin and further a releasing oil such as silicone oil is applied to the surface of the fixing roller. Although this countermeasure is extremely effective to prevent toner offset, a device to supply a release oil is extra required, thereby increasing the size of a fixing device. Therefore, this is not suitable in terms of reduction in size as a whole. Therefore, as for a monochrome toner, another method is instead adopted in which no or a little amount of release oil is applied to a fixing roller (hereinafter referred to as oilless method). In such an oilless method, viscosity and elasticity of a melted toner are increased by adjusting molecular weight distribution of a binder resin to prevent inside rupture of the melted toner and further a release agent such as wax is contained in the toner.
In addition, oilless methods are also increasingly adopted for color toners as well as monochrome toners in terms of the size reduction and simplification of a machine. However, as mentioned above, in the case of a color toner, it is necessary to smooth the surface of an unfixed image to improve the color reproduction. Therefore, it is inevitable to reduce the viscosity and elasticity of a toner during melting. That is, relative to the case of a monochrome toner having a relatively low gloss, a color toner tends to offset so that it is difficult to adopt the oilless method mentioned above for a fixing device. In addition, when a release agent is contained in a toner, the attachment property of the toner is strengthened. As a result, the transferability of the toner to a transfer medium deteriorates. Further, this causes a problem that the release agent contained in the toner contaminates a friction charging member such as a carrier and reduces the chargeability of the friction charging member, resulting in deterioration of the durability of the friction charging member.
On the other hand, a carrier has a hard coating layer formed of a suitable resin material to prevent filming of toner components on the surface of the carrier, oxidization of the surface of the carrier, deterioration of humid sensitivity of the carrier, and the attachment of the carrier to the surface of an image bearing member, to prolong the life of a developer containing the carrier, to protect an image bearing member from being scratched or abraded by the contact with the carrier, and to control the charging polarity or adjust the amount of charge in the carrier. For example, unexamined published Japanese patent application No. (hereinafter referred to as JOP) 58-108548 describes a carrier covered with a specific resin material. JOP 54-155048, 57-40267, 58-108549, 59-166968, and H6-202381, and examined published Japanese patent applications Nos. (hereinafter referred to as JPP) H3-628 and H119584 describe a coating layer of a carrier to which various kinds of additives are attached. JOP 5-273789 describes a carrier, to the surface of which an additive is attached. JOP H9-160304 describes a carrier having a coating layer which contains electroconductive particles having a diameter larger than the thickness of the coating layer. In addition, JOP H8-6307 describes a carrier having a coating layer mainly formed of a benzoguanamine-n-butyl alcohol-formaldehyde copolymer and Japanese Patent No. 2683624 describes a carrier having a coating layer formed of a cross-linkage compound of a melamine resin and an acrylic resin.
However, these carriers do not still have sufficient durability and cannot sufficiently restrain carrier attachment. Problems related to the durability are, for example, toner spent on the surface of the carrier, unstable charging state due to the toner spent, reduction in the thickness of coating layer due to scraping of the coated resin, and decrease in resistance due to the reduction in the thickness of the coated resin. Initially good images can be obtained but as the number of copies increases, quality of the images obtained deteriorates. This is a problem to be solved.
In addition, as the demand to make it faster and more beautiful is stronger, the speed of machines has recently become significantly faster. With this increase in the speed of machines, the stress on a developer greatly increases. Thereby, the life length of a carrier, which is normally sufficiently long, becomes short for a practical use. Further, carbon black has been typically used as a resistance adjuster for a carrier in many cases. This creates a concern that carbon black may transfer to a color image and cause color contamination due to layer scraping and/or detachment of carbon black. Various kinds of countermeasures have been proposed and have effects on prevention of such color contamination.
For example, JOP H07-140723 proposes a carrier including electroconductive material (i.e., carbon black) present on the surface of the core material but not in a resin coating layer. In addition, JOP H08-179570 proposes a carrier including a resin coating layer having a density gradient. The density thereof goes thinner toward the surface of the resin coating layer and carbon black is not present on the surface of the resin coating layer. Further, JOP H08-286429 proposes a double-layer coating type carrier which has an inner coating layer containing electroconductive carbon on the surface of a core particle and another layer, i.e., surface coating resin layer, containing white color electroconductive material, on the inner coating layer. However, these carriers cannot deal with high stress on developers and thus the color contamination problem remains unsolved.
It is apparent that removing carbon black, which is a root cause of color contamination, is most effective to solve this color contamination problem. However, since carbon black has a low electric resistance, the resistance of a carrier rises when carbon black is simply removed. Commonly, when a carrier having a high electric resistance is used in a developer for a photocopying image having a large area, obtained images have a sharp edge effect, meaning that the image density is extremely thin at the center portion and thick only at the edge portion.
In addition, when an image is formed of characters and fine lines, a vivid image is obtained because of this edge effect. But when an image has an intermediate tone, there is a drawback in that obtained images have extremely poor reproducibility.
Generally, as resistance adjuster other than carbon black, for example, titanium oxide and zinc oxides are known. However, these compounds do not have the same effect as carbon black with regard to lowering the resistance of a carrier. This problem remains unsolved.